At what point did Ivan's marriage begin to decline?

Answers
1

This happens after they move to Petersburg. He throws himself into making money and buying fine things. These misplaced priorities are shared by the whole family, and show up in Iván Ilych's marriage. Tolstoy mentions a particularly violent quarrel between Iván Ilych and his wife, in which they argue over some surplus pastries he ordered. During the fight, Iván Ilych makes "angry allusions to divorce" (134). What a fine marriage it is, when divorce is brought up in an argument over pastries!